The present invention relates to a cleaning method using the generation of cavitation in which bubbles in liquid are eliminated by ultrasonic waves, and thus, the generation of cavitation becomes easy, such that the cleaning effect is improved.
Generally, in a cleaning method using an ultrasonic wave, cavitation is generated due to the ultrasonic wave emitted in liquid and the stain on the thing to be cleaned is removed from it due to an impulse wave form the generation of cavitation. When the ultrasonic wave is emitted in the liquid, small gaseous bodies dissolved in the liquid are compressed and expanded, thus generating the cavitation in a liquid. Accordingly, cleaning may be executed by impulse waves which are generated due to generation and elimination of cavitation in the liquid.
In a cleaning apparatus (see FIG. 1) using an ultrasonic wave of one frequency, when a signal of one frequency from an oscillator 3 is supplied to a vibrator 2 and the ultrasonic wave of the one frequency is generated from the vibrator 2 attached to a cleaning tank 1, cavitation generates in portion corresponding to large amplitudes of the standing wave as shown at A.
In such a cleaning method, however, because cavitation is not generated in the small amplitude portion of the standing wave, cleaning is not uniformly performed. Also, in such method, because the ultrasonic wave is not transmitted to all areas of the tank 1 due to the cavitation in the large amplitude portion of standing wave, the cavitation is not effectively generated in all areas in the tank 1.
For solving such defect, the applicant provided an asymmetric Langevin type vibrator 8 in which piezoelectric vibrators 7 and electrode 7a and 7b are put between a long metal block 5 and a short metal block 6 and screw threads at both ends of a bolt are engaged with screw threads of the metal blocks 5 and 6 (see FIG. 2).
This vibrator 8 can generate ultrasonic waves having a resonance frequency f.sub.1 of a length between the long metal block 5 and the piezoelectric vibrator 7. A resonance frequency f.sub.2 of a length between the short metal block 6 and the piezoelectric vibrator 7 and a resonance frequency f.sub.3 of all length of the vibrator 8.
As shown in FIG. 3, when signals of frequencies f.sub.1, f.sub.2 and f.sub.3 are applied to the vibrator 8 from every predetermined time period ultrasonic waves oscillators 10, 11 and 12 by switching a switch 9 having frequencies f.sub.1, f.sub.2 and f.sub.3 are respectively generated from the vibrator 8 every predetermined time period. Because the positions of large amplitudes of the ultrasonic waves are different from each other as shown in dotted lines A, cavitation can be generated in different positions of liquid 4 in the tank 1. Therefore, the cleaning effect in this cleaning method is improved in comparison with the cleaning method of the one frequency.
In this cleaning method, however, because cavitation is not generated between the dotted lines A in the liquid 4, cleaning is not uniformly performed. Because cavitation is generated in the position of the large amplitudes in the ultrasonic wave when a standing wave is generated with the ultrasonic wave of one frequency, an additional supply of power of the ultrasonic wave is restrained by the cavitation.
When the ultrasonic wave is changed to another frequency and the pattern of the standing wave is changed, cavitation generated with the ultrasonic wave of one frequency is scattered with the ultrasonic wave of the other frequency. Then, cavitation remains in the position of the large amplitude of the standing wave in the next ultrasonic wave and it becomes the origin in the next cavitation.